plugin: git (toggle skip ci)

[pauby]

need a flag to toggle [skip ci]

Use case:
If CI (example Jenkins) is completely skipped with this flag, and a gitrepo is updated in a way unbeknownst to the CI server, then the CI server's concept of PREVIOUS_COMMIT will be the last commit it knows about.

In the case where [skip ci] is used, it would be the commits before AU ran, instead of the actual last commit.

So in this case the workaround is to allow AU to run thru CI/CD tools, and have a conditional that checks to see if this is an AU commit and noop it, providing a functionally equivlant outcome.

Issue migrated from majkinetor/au#197